Secondary Glazing for Sash Windows

Three separate paths are at work. First, the glass itself: 4mm float has a critical frequency that coincides with traffic noise, so it re-radiates rather than blocks. Second, the perimeter gaps at the meeting rail, staff bead and parting bead, which typically total 200-400cm² of open area. Third, the pulley stile boxes, which connect the room directly to the outside via the weight cavity.

Replacement double glazing addresses only the first path, which is why homeowners who replace sashes for acoustic reasons are so often disappointed. Secondary glazing addresses all three at once by placing a sealed independent barrier across the whole reveal.

Performance is a product of three variables: glass mass, cavity depth and seal integrity. Skipping any one of them caps the result. Our standard high-performance sash specification is 10.8mm acoustic laminated glass (two panes bonded with an acoustic PVB interlayer), set at 150mm from the original sash, with compression EPDM seals on all closing edges.

The interlayer matters more than raw thickness. Acoustic PVB damps the coincidence dip that plain laminate suffers from, which is worth 4-5dB in exactly the frequency band where traffic and voices sit.

Vertical sliders mirror the movement of the original sash and are the intuitive choice on tall Georgian boxes. Horizontal sliders are lighter to operate and generally preferred on Victorian two-over-two sashes. Where shutters are in use, hinged units swing clear of the shutter box.

Because nothing is removed and all fixings go into the reveal lining rather than the original joinery, secondary glazing is the only glazing upgrade routinely approved on Grade I and Grade II listed sash windows. Historic England explicitly identifies it as a reversible, conservation-compatible intervention.

A traditional London box sash is a machine: two sashes, two cords, two weights and a pulley stile that has to stay clear. Any secondary system that pins, packs or screws into that assembly stops it working and damages the part of the window that is genuinely historic. We never fix into the sash, the pulley stile or the staff bead. The secondary frame lands on the reveal or, where the reveal is shallow, onto the architrave with a made-up sub-frame.

Because the original sashes keep sliding, the secondary unit is normally a vertical slider so both layers behave the same way — you lift the secondary pane, then lift the sash behind it. That pairing is the reason sliding secondary glazing is the delivery route for almost every sash installation we do in London.

Most sash windows in London leak air at the meeting rail and around the perimeter of the sash. That leakage is why an original sash typically measures somewhere around 25-30dB reduction on its own, well below what the glass alone would predict — sound follows air. Sealing the secondary layer removes the leakage path entirely, which is where the large jump in perceived quiet comes from.

With a 100-150mm cavity and 6.4mm acoustic laminate we would expect a total reduction in the low to mid 40s dB; with 10.8mm laminate and a generous gap, up to 54dB in suitable configurations. What we will not promise is a specific figure before a survey, because the achievable cavity in a shallow Georgian reveal and a deep Victorian one are not the same window problem.

Sash windows that are already failing should be repaired before secondary glazing goes in front of them, not after. Our surveyor records cord condition, cill and bottom rail decay, paint build-up on the pulley stile and whether the sashes still meet cleanly. Small repairs are cheap now and expensive once a secondary frame is in the way.

The upside for period owners is that a sealed secondary layer keeps warm moist room air off the cold primary glass, so the running condensation that rots bottom rails and cills largely stops. Owners of Georgian and Victorian sashes usually get more long-term value from that than from the dB figure.